Is it fair to say that aggregator is retained in d7 core for legacy support, and that Feeds is the likely successor? Feeds covers more scenarios than aggregator, and appears to be better situated for adaptions, although the generalization makes it more difficult to understand how to go about implementing. Are there projects that you can recommend that build on themes to implement particular use cases?
--- Looking through the Feeds documentation, we find the statement
"Feed: aggregation importer. Aggregates RSS/Atom feeds to nodes. Provides a node type Feed and a node type Feed item. Create one or more "Feed" nodes to add RSS/Atom feeds to your site. On cron, these feeds will continuously produce "Feed item" nodes. Requires the Aggregator core module to be enabled.
So project/feeds depends on core aggregator, although it seems to overlap and extend functionality as well.